1. Field of the Invention
This invention relates to light absorbing materials and, more particularly, to materials useful for absorbing solar energy.
2. Art Background
The collection of energy derived from solar radiation is an alluring prospect. One method contemplated for solar energy utilization is the production of heat through absorption of solar energy by an absorbing, i.e., black material, utilized in a solar collector. Collectors are typically fabricated by depositing an absorbing material on a substrate that is an efficient heat conductor. Solar energy is directed onto the absorber by an optical system. The heat produced by absorption is conducted through the substrate and is either exchanged with a heat transfer medium or used directly.
A variety of materials has been proposed as solar absorbers. (See C. M. Lambert, Solar Energy Materials, 1, 319 (1979).) Exemplary of these absorbing materials is electroplated black chrome (a complicated mixture of chrome and chrome oxides), an evaporated platinum-aluminum oxide mixture, and a dendritic tungsten material (described in Solar Energy, 17, 119 (1975), Solar Energy Materials, 1, 105 (1979), and Applied Physics Letters, 26, 557 (1975), respectively). Although each of these exemplary materials has desirable properties, each also has some limitations.
The use of solar concentrators (the focusing of solar radiation onto an absorbing material) has been contemplated to increase the efficiency of heat production and to yield higher temperatures for directly driving chemical reactions. Below 300 degrees C., the chrome mixture and the dendritic tungsten typically are useful. However, at increased temperatures both materials degrade. The chrome/chrome oxide compositions undergo decomposition induced by temperatures above 300 degrees C. The tungsten materials are stable in an inert atmosphere above 300 degrees C., but seriously degenerate at these temperatures in the presence of an oxidizing medium such as air. Thus, although most of the newer absorbing materials appear useful for solar energy absorption of one sun, at higher sun densities--temperatures experienced when solar concentration is employed--they exhibit significant problems. The platinum/aluminum oxide composite exhibits better stability, but is generally not useful above approximately 500 degrees C.
Beside the difficulties associated with solar concentration, many of the absorbing materials including those previously discussed have acceptable absorption efficiencies, but re-radiate a substantial portion of the absorbed energy. This re-radiation results in decreased solar conversion efficiency. Additionally, presently available materials, such as the evaporated platinum/aluminum oxide mixture, are expensive and severely limit the applications for which solar energy is economical.